The present invention relates to a fuel cell of the stack type in which power generation cells are stacked, and a method of operating the same. More particularly, the invention relates to a fuel cell in a stacked arrangement which includes an air supply/exhaust mechanism capable of sequentially removing water generated at air poles on a cell-by-cell basis, and a method of operating same.
Fuel cells can produce electrical energy from an electrochemical reaction between hydrogen and oxygen have been demanded to be enhanced in function and be further reduced in size and weight. Such fuel cells are desired as an alternative to fossil fuel energy systems which are typically used at present, such as internal-combustion engines. Since the voltage obtained from unit power generation cell of the power generation cells constituting a fuel cell is low, the fuel cell is generally arranged into a stack of a plurality of sheets of power generation cells and is so designed that the requisite amount of electrical energy can be obtained.
Generally, the power generation cells have gas conduits disposed in parallel relative to supply pipes for supplying hydrogen gas, which is a fuel, and oxygen gas, and the hydrogen gas and the oxygen gas are supplied through respective gas feed pipes for supplying the hydrogen gas and air. Also, the gas exhausted from the power generation cells is discharged into an exhaust pipe through gas conduits disposed in parallel relative to the exhaust pipe.
FIG. 6 is a schematic diagram showing the structure of a fuel cell including an air supply/exhaust device of the conventional type, and is a schematic structural diagram showing only the air supply side. Air is supplied into gas conduits 102 connected in parallel to an air supply pipe 103. Further, air is supplied through the gas conduits 102 into power generation cells 101, and the air having passed through each power generation cell 101 is discharged to the exterior through an air exhaust pipe 104. FIG. 7 is a schematic diagram of the structure of a fuel cell including an air supply/exhaust device of the conventional type shown in FIG. 6. The fuel cell having the conventional type air supply/exhaust device is generally constituted of four kinds of elements, namely, an air supply pipe 103, a plurality of power generation cells 101, gas conduits 102 and an air exhaust pipe 104, and air is simultaneously supplied from the air supply pipe 103 into each power generation cell 101, whereby the requisite amount of electrical energy can be taken out.
It is important for an energy supply system to be small in size, high in performance and so designed that degradation of performance during operation is restrained. When a fuel cell of the above-mentioned structure is used as an energy supply system, the reaction between hydrogen and oxygen results in formation of water as a by-product on the air pole side where oxygen is supplied, and the flow of air at the time of supplying oxygen-containing air to the power generation cells may be thereby hindered. Therefore, it is important to smoothly supply air into the gas conduits and to remove water.
There may be cases where, in the process of continuously performing power generation, water as the by-product stands as an obstacle to the flow of air through the gas conduits, so that stable power generation is not achieved and electrical energy is not taken out sufficiently. For achieving stable power generation, it may be contemplated to supply a sufficient quantity of air from the air supply pipe 103 into the gas conduits 102. However, when the number of the power generation cells 101 stacked is increased, the ratio of the quantity of air supplied into one power generation cell 101 to the quantity of air supplied from the air supply pipe 103 becomes small, so that much air must be supplied from the air supply pipe 103 into the gas conduits 102 in order to discharge water assuredly. In order to supply a sufficient quantity of air from the air supply pipe 103 into the gas conduits 102 so as to remove the water, the component parts constituting the fuel cell and the like are necessarily enlarged in size; thus, it has been difficult to manufacture a fuel cell small in size and high in performance.
In order to securely discharge the water accumulated in the gas conduits 102 by supplying air, as shown in FIG. 8, valves 105 operated independently on the basis of each power generation cell 101 are provided at inlets of the gas conduits 102, and the valves 105 are opened and closed in a time-sharing manner. When air is supplied on the basis of each power generation cell 101 by opening and closing the valves 105, it is possible to supply air only to the relevant power generation cell 101 at the time when the valve 105 is opened, whereby water can be discharged. However, arrangement of the valves 105 for on the basis of each power generation valve 101 can provide a complicated structure and in increase in the number of component parts. Further, the power generation cells 101 corresponding to the closed valves 105 are not supplied with air, and, therefore, stable power generation cannot be achieved.
In addition, there may be contemplated an air supply system in which, as shown in FIG. 9, bypass lines 106 are provided in parallel to the valves 105 so as to constantly supply a fixed quantity of air, whereby the fixed quantity of air is supplied to all the power generation cells 101 while discharging water. There may also be contemplated another air supply system in which, as shown in FIG. 10, the air supply pipe 103 is divided into two systems, namely, air supply pipes 103a and 103b to thereby supply air. With these air supply systems, it is possible to constantly supply a sufficient quantity of air to all the power generation cells 101 and to remove water generated by association of hydrogen and oxygen from the gas conduits 102. However, these air supply systems are complicated in structure and are therefore not suited to a fuel cell for practical use. Besides, in the case of providing a greater quantity of electrical energy, the amount of water accumulated in the gas conduits 102 is also increased, so that it becomes difficult for air to flow smoothly in the gas conduits 102, thus making it very difficult, if not impossible, to achieve stable power generation.